The above entitled patent applications describes a method of laminating polyimide to thin sheet metal, such as copper and to a stainless steel substrate. As disclosed in the above-entitled patent applications, a composite laminate is made by providing a stainless steel substrate to which a solution of an intractable polyimide precursor is applied and then dried to form a tack-free film. Thereafter, a solution of the precursor of a thermoplastic polyimide is applied to the film of intractable polyimide precursor and dried to form a tack-free second film. Both the films are then cured concomitantly at a sufficiently rapid rate, preferably with IR radiation, to effect near complete imidization of the polyimide precursors of both films. Thereafter, a conducting metal sheet or foil, such as a beryllium copper or other copper alloy is laminated onto the thermoplastic polyimide film according to the process disclosed therein. This will provide a composite laminate material comprised of a relatively thick rigid stainless steel substrate, a layer of an intractable polyimide bonded thereto, a layer of thermoplastic polyimide bonded to the intractable polyimide, and a sheet of thin conducting material such as copper foil bonded to the thermoplastic polyimide. In the above noted patent application Ser. No. 07/695,850, the intractable polyimide is disclosed as being preferably poly(4,4'-oxydiphenylene benezene-1,2,4,5, tetracarboxylicdiimide) often referred to as PMDA-ODA of which Pyralin (a registered trademark of E.I. DuPont) 5878 manufactured by E.I. DuPont is an example and the thermoplastic polyimide layer as disclosed as being preferably poly(4,4'-oxydiphenylene 1,1'-hexafluoropropyldiphenylene 3,3' , 4,4'-tetracarboximide) also often referred to as 6FPDA-ODA, of which Pyralin 2566 manufactured by E.I. DuPont is an example.
One of the uses of this laminate is in forming suspension arm assemblies in direct access storage devices (DASD's) such as the type shown in U.S. Pat. No. 4,996,623. In such applications it is typically required that the copper be etched to form circuit patterns and that the stainless steel be removed by etching at a given location across the strip in order to provide a hinge so that the composite can be bent to a given angle. In the formation of this hinge, the properties of both the polyimide layers and the copper layers are critical. For example, if the copper layer is too thick, the dynamic performance of the arm will suffer. Moreover, the polyimide layers must be sufficiently thin (i.e. a total thickness of less than about 10 .mu.m) to allow the copper to control the properties of the hinge. It is desirable that the composite material remain flat after all of the etching has taken place so that when the bend angle for the hinge is formed, it can be accurately formed starting with a flat sheet of material. (The etching referred to includes both forming the circuit on the copper foil and removal of the stainless steel in the hinge region.) It has been found that the composite in fact does remain flat as long as the stainless steel backing material or substrate remains intact. However, it was found with the use of the materials and their thicknesses as disclosed in the above entitled patent application, when the stainless steel was etched to form a hinge, unbalanced residual stresses caused a bending of the composite material at the location where the stainless steel was removed, thereby rendering a curved rather than a flat material at the hinge location. Typically, curving would cause a configuration which was concave on the side of the stainless steel. It is believed that this bending or flexing is due to a combination of factors including different coefficients of thermal expansion (CTE) of the two polyimides themselves, between the polyimides and stainless steel, and between the polyimides and copper, resulting in an unbalanced thermal stress being created due to the heating and cooling during the laminating cycle. Briefly explained, it is believed that the stress develops because each of the polyimides has a CTE (36-45 ppm/.degree. C.) which is greater than the CTE (17-20 ppm/.degree. C.) of each the copper and stainless steel. During the curing processes and the lamination when these materials are heated they expand before the bonding takes place, but once the bonding has taken place and the materials cool there are unbalanced stresses which result principally at polyimide to polyimide and also the polyimide to copper foil interfaces. As long as the relatively thick stainless steel is tightly bonded to the intractable polyimide the thickness and rigidity of the stainless steel together with the nature and direction of the unbalanced forces prevent any appreciable flexure or strain as a result of the unbalanced stresses. However, once the stainless steel is removed by etching to form the hinge region, this solid relatively thick stainless steel backing is no longer available and due to the relatively thin films of the polyimide and the thin copper metal, the unbalanced stresses (especially at the copper-polyimide interface) result in a strain in the form of a flexure or curving of the composite at the area where the stainless steel has been removed.
While thicker polyimide coatings and thicker copper material could alleviate this condition to some extent, such increased thicknesses are not feasible because of performance requirements in suspension arms for DASD's as described above. Thus, there is a need to provide a composite material suitable for suspension arms for DASD wherein the hinge can be formed starting with a relatively flat composite even after the stainless steel substrate has been removed to form a hinge region.